1. Field of the Invention
This invention pertains to the measurement of fluid flow in a conduit, with compensation for variations in the temperature and density of the fluid. The invention has a wide range of specific applications, including, for example, monitoring of blood flowing outside the body of a patient through tubing during medical procedures, measurement of the velocity of a ship or other vessel traveling through water, monitoring of the flow of petroleum materials to a flare stack in a petroleum production or refining facility, measurement of the flow of air or fuel to an engine, measurement of the flow of hydrocarbon fluids from a well head, storage tank or pump, and measurement of fluid flow in a chemical-carrying conduit. In addition, the invention has general applicability in many fields, including, for example, medicine, biology, automobiles, aerospace, the military, oceanography, meteorology, cryogenics, pharmacology, chemical production and processing, agriculture, and the oil and gas industry.
2. State of the Art
With reference to FIG. 1A, a well-known technique for measuring the flow (denoted by arrows) of a fluid 8 through a conduit 10 is commonly referred to as xe2x80x9ctransit timexe2x80x9d flow metering. In this technique, transducer driving signals 12 and 14 cause a downstream ultrasonic transducer 16 to emit an ultrasonic signal 18 that traverses the conduit 10 and the fluid 8 and is received by an upstream ultrasonic transducer 20. Because the ultrasonic signal 18 takes time to pass through the conduit 10 and the fluid 8, transducer output signals 22 and 24 indicative of the received ultrasonic signal 18 lag behind the transducer driving signals 12 and 14 by an upstream phase shift xcex94xcfx86upstream.
As shown in FIG. 1B, every few milliseconds, the roles of the upstream and downstream transducers 20 and 16 are reversed so that the transducer driving signals 12 and 14 cause the upstream transducer 20 to emit an ultrasonic signal 28 that traverses the conduit 10 and the fluid 8 and is received by the downstream transducer 16. Because the ultrasonic signal 28 takes time to pass through the conduit 10 and the fluid 8, transducer output signals 30 and 32 indicative of the received ultrasonic signal 28 lag behind the transducer driving signals 12 and 14 by a downstream phase shift xcex94xcfx86downstream.
When the fluid 8 is not flowing, the upstream and downstream phase shifts xcex94xcfx86upstream and xcex94xcfx86downstream equal one another and are attributable solely to properties of the fluid 8 other than flow (primarily temperature, density, and compressibility). Thus, under this condition,
xcex94xcfx86upstream=xcex94xcfx86downstream=xcex94xcfx86fluidxe2x80x83xe2x80x83(1)
where xcex94xcfx86fluid is the phase shift attributable to properties of the fluid 8 other than flow.
When, instead, the fluid 8 is flowing, the upstream and downstream phase shifts xcex94xcfx86upstream and xcex94xcfx86downstream do not equal one another. Rather, the upstream phase shift xcex94xcfx86upstream includes an additional component xcex94xcfx86upxe2x80x94flow attributable to the additional time the ultrasonic signal 18 takes to traverse upstream against the flowing fluid 8, while the downstream phase shift xcex94xcfx86downstream is reduced by a component xcex94xcfx86downxe2x80x94flow attributable to the ultrasonic signal 28 being aided by traversing downstream with the flowing fluid 8. Thus,
xcex94xcfx86upstream=xcex94xcfx86fluid+xcex94xcfx86upxe2x80x94flowxe2x80x83xe2x80x83(2)
xcex94xcfx86downstream=xcex94xcfx86fluidxe2x88x92xcex94xcfx86downxe2x80x94flowxe2x80x83xe2x80x83(3)
The components xcex94xcfx86upxe2x80x94flow and xcex94xcfx86downxe2x80x94flow may be defined as follows:
xcex94xcfx86upxe2x80x94flow=(2xcfx80fxc3x97s)/(c+(rfxc3x97cos(xcex8)))xe2x80x83xe2x80x83(4)
xcex94xcfx86downxe2x80x94flow=(2xcfx80fxc3x97s)/(cxe2x88x92(rfxc3x97cos(xcex8)))xe2x80x83xe2x80x83(5)
where f is the frequency of the ultrasonic signals 18 and 28, s is the distance shown in FIGS. 1A and 1B between the transducers 16 and 20, c is the speed of sound in the fluid 8, rf is the rate of flow of the fluid 8 in the conduit 10, and the angle xcex8 is the angle shown in FIGS. 1A and 1B between the axis of transmission of the ultrasonic signals 18 and 28 and the longitudinal axis of the conduit 10.
As shown in FIG. 1C, taking the difference between the downstream phase shift xcex94xcfx86downstream and the upstream phase shift xcex94xcfx86upstream using a differential amplifier 34 cancels out the phase shift xcex94xcfx86fluid and yields the sum of the phase shift components xcex94xcfx86upxe2x80x94flow and xcex94xcfx86downxe2x80x94flow as the output xcex94xcfx86flow of the amplifier 34. Thus,
xcex94xcfx86upstreamxe2x88x92xcex94xcfx86downstream=xcex94xcfx86fluid+xcex94xcfx86upxe2x80x94flowxe2x88x92(xcex94xcfx86fluidxe2x88x92xcex94xcfx86downxe2x80x94flow)=xcex94xcfx86flowxe2x80x83xe2x80x83(6)
=xcex94xcfx86upxe2x80x94flow+xcex94xcfx86downxe2x80x94flow=xcex94xcfx86flowxe2x80x83xe2x80x83(7)
Since the phase shift components xcex94xcfx86upxe2x80x94flow and xcex94xcfx86downxe2x80x94flow are related to fluid flow rf (see equations (4) and (5) above), it may be said that
rf=ƒ(xcex94xcfx86flow, f, s, c, xcex8)xe2x80x83xe2x80x83(8)
Thus, with appropriate correlation of the output xcex94xcfx86flow of the differential amplifier 34 to the fluid flow rf using calibrated circuitry, the fluid flow rf may be determined.
A conventional transit time flow metering system like that discussed above is described in more detail in U.S. Pat. No. 4,227,407 to Drost. Also, applications for conventional transit time flow metering systems like that described above are found in a wide variety of contexts, including measuring petroleum materials flowing to a flare stack, as described in U.S. Pat. No. 4,596,133 to Smalling et al., measuring air flowing to an automobile engine, as described in U.S. Pat. No. 4,488,428 to Taniuchi, and monitoring blood flowing outside the body of a patient (xe2x80x9cextracorporeal blood flowxe2x80x9d) through tubing during medical procedures to actuate a clamp on the tubing, if necessary, to prevent back-flow flow of the blood, as described in U.S. Pat. No. 5,445,613 to Orth (assigned to the Assignee of the present invention, Rocky Mountain Research, Inc. of Salt Lake City, Utah).
While conventional transit time flow metering systems are useful in a variety of contexts, they traditionally lack the accuracy necessary or desirable in some instances. For example, while state-of-the-art transit time flow metering systems can be accurate to within xc2x12%, some applications, like extracorporeal blood flow monitoring, would benefit from accuracies within xc2x11%. Transit time flow metering systems traditionally lack greater accuracy because the above-described process of correlating the value xcex94xcfx86flow to fluid flow is subject to errors resulting primarily from variations in the temperature and density of the fluid being measured.
Therefore, there is a need in the art for a transit time ultrasonic fluid flow metering apparatus and method with compensation for variations in the temperature and density of the fluid for enhanced accuracy. Such an apparatus and method should have applicability in a wide variety of flow metering contexts.
An inventive transit time flow meter includes an assembly for transmitting an ultrasonic signal through fluid in a conduit and for receiving the transmitted ultrasonic signal. The assembly may comprise a pair of ultrasonic transducers. Circuitry coupleable to the transmitting and receiving assembly detects phase shifts in the received ultrasonic signal relative to the transmitted ultrasonic signal, and circuitry coupled to the phase shift detecting circuitry adjusts future detection of such phase shifts in response to already-detected phase shifts to enhance the accuracy of such future detection. Thus, the present invention uses feedback during the detection of phase shifts indicative of the flow of the fluid in the conduit to enhance future detection of such phase shifts and thereby enhance accurate correlation of the phase shifts to fluid flow.
As an example, the phase shift detecting circuitry may include an exclusive-OR (XOR) phase comparator having a limited range of relatively high accuracy, such as between 45xc2x0 and 135xc2x0. The adjusting circuitry then includes feedback circuitry for maintaining the phase shifts detected by the XOR phase comparator within its limited range to enhance the accuracy of the comparator""s detections, thus enhancing accurate correlation of the detected phase shifts to fluid flow.
An inventive apparatus for conducting a fluid while metering flow of the fluid includes a fluid conduit through which the fluid may flow and the above-described flow meter. The conduit may be, for example, tubing (e.g., Tygon(copyright) tubing), a water conduit on a hull of a vessel, a flare stack inlet pipe, an air inlet of an engine, a fuel inlet of an engine, a pipeline, a drilling rig conduit, or a conduit used in a chemical process.
In a preferred embodiment, an extracorporeal blood flow system includes a blood conduit connectable to a patient for diverting blood from, and later returning blood to, the patient, a pump connectable in-line with the blood conduit for pumping blood therethrough, and the flow meter described above. The system may also include one or more powered clamps attached to the blood conduit for clamping the conduit closed.
In another embodiment, flow metering circuitry for use in a flow metering apparatus having the ultrasonic transmitting and receiving assembly described above includes circuitry for starting a transit time count upon the initiation of transmission of an ultrasonic signal through the fluid and for stopping the transit time count upon reception of the transmitted ultrasonic signal. The count circuitry may be, for example, a digital counter. The flow metering circuitry may also include circuitry for correlating transit time counts by the count circuitry to flows or temperatures of the fluid, thus allowing for determination of the fluid flows or the temperature of the fluid.
In still another embodiment, an apparatus for detecting anomalies, such as air bubbles in a fluid such as blood flowing in a conduit, includes the above-described ultrasonic transmitting and receiving assembly. Circuitry coupleable to the transmitting and receiving assembly outputs an anomaly detection signal that changes relatively rapidly in response to variations in the amplitude of the received ultrasonic signal. Such circuitry may include an amplifier and a Resistor-Capacitor (RC) network. Sample-and-hold circuitry coupleable to the transmitting and receiving assembly samples the amplitude of the received ultrasonic signal and outputs another anomaly detection signal that changes relatively slowly in response to variations in the amplitude. Circuitry coupled to the rapidly changing anomaly detection signal outputting circuitry and the sample-and-hold circuitry then detects differences between the rapidly changing and slowly changing anomaly detection signals indicative of anomalies in the fluid.
Various methods of the present invention correspond to the structures described above. One such methodxe2x80x94a method of measuring the temperature of a fluid in a conduitxe2x80x94includes transmitting an ultrasonic signal through the fluid. The ultrasonic signal is then received, and phase shifts in the received signal relative to the transmitted signal are detected. The detected phase shifts are then correlated to the temperature of the fluid, and the detection of the phase shifts is adjusted in response to previously detected phase shifts to enhance the accuracy of the correlation of detected phase shifts to the temperature of the fluid.